1. Field
The present disclosure is directed to a method and apparatus for radio link failure recovery. More particularly, the present disclosure is directed to acquiring system information prior to radio link failure.
2. Introduction
Presently, wireless communication devices, such as cellular phones, personal digital assistants, cellular wireless equipped laptop computers, and other wireless communication devices, provide mobile communication ability for users. Unfortunately, a wireless communication device can drop a connection to a cellular network when it experiences radio link failure. A radio link failure occurs when a wireless communication device that is connected to a cell experiences a sudden deterioration of the signal such that normal communication can no longer be maintained. The deterioration occurs so rapidly that the link between the wireless communication device and the serving cell fails before the wireless communication device can be handed over to another cell. In a typical cellular deployment, radio link failure occurs when a wireless communication device suddenly experiences shadowing, such as when the wireless communication device turns a corner and the signal from the serving cell is obstructed by a building.
Radio link failure can also be caused by a sudden increase in interference from neighbor cells accompanied by shadowing of the serving cell. This is a significant problem in a heterogeneous network, where macro cells, femtocells, and picocells overlap on the same carrier frequency. For example, the femtocells and picocells can be deployed in homes and offices for preferential services to users. A wireless communication device connected to a macro cell can experience a sudden increase in interference from a femtocell or picocell, for example, when the user enters a building where the femtocell or picocell is located. Femtocell and picocell deployments can ultimately be quite common, such as with the introduction of long term evolution closed subscriber group cells, and there can be hundreds of such cells within the coverage of a macro cell. As a result, a wireless device is more likely to experience radio link failure in a heterogeneous network than in a conventional network.
Presently, when a wireless communication device experiences a radio link failure, it goes through a connection reestablishment procedure. Unfortunately, the current procedure has drawbacks. One drawback is the delay in reestablishment. For example, the current contention based random access procedure has significant delays due to the need to perform contention resolution. Moreover, the wireless communication device has to first acquire system information of the target cell to perform random access, and this can add significant delay to the recovery procedure. Another drawback is that the current reestablishment procedure does not adequately support home cells, such as femtocells or picocells. For example, the general assumption is that only cells at the same base station are prepared for handover. This implies that radio link failures in some scenarios involving home cells in heterogeneous networks are unrecoverable. A further drawback is that there is no mechanism to forward pending data from the source cell to a reestablishment target. This results in a loss of a substantial amount of user data. Furthermore, if reestablishment is to occur on a cell that is on a different frequency, such as when the primary interferer is a closed subscriber group cell to which the wireless communication device does not have access, there is currently no mechanism to trigger the system information acquisition of the target cell.
Thus, there is a need for an improved method and apparatus for radio link failure recovery.